Lubrication Structure of Cranking Rotational Force Transmission Mechanism for Internal Combustion Engine

ABSTRACT

The lubricant-return holes ( 12   d ) are provided in the intermediate portion ( 12   c ) of the ring gear ( 12 ). Thus, the lubricant stagnating above the first oil sealing member ( 32 ) can be easily discharged to the oil pan ( 36 ) side via the lubricant-return holes ( 12   d ) even when the ring gear ( 12 ) has stopped rotating in response to the starter motor being turned off after completion of the start of the internal combustion engine. As such, the level of the surface of lubricant stagnating between the ring gear ( 12 ) and the outer race member ( 14 ) does not rise, and therefore the lubricant surface virtually does not reach the outer race ( 30 ) and the one-way clutch ( 20 ) that are rotating while the internal combustion engine is running. Thus, bubbling of lubricant and production of sludge, which may otherwise be caused by agitation of the lubricant surface, can be prevented.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a lubrication structure of a crankingrotational force transmission mechanism for an internal combustionengine, which transmits the rotational force of a starter motor in onerotational direction to a rotational output shaft of the internalcombustion engine via a one-way clutch and interrupts the transmissionof the rotational force of the starter motor in the other rotationaldirection via the one-way clutch.

2. Description of the Related Art

As a cranking rotational force transmission mechanism for an internalcombustion engine (e.g., an internal combustion engine for a vehicle)which transmits the rotational force of a starter motor to thecrankshaft (i.e., rotational output shaft) of the internal combustionengine, JP-A-2000-274337 describes a mechanism in which the ring gearand the pinion gear on the starter motor side are arranged to beconstantly in mesh with each other.

In such a constantly-meshed type cranking rotational force transmissionmechanism for an internal combustion engine, a one-way clutch isprovided between the ring gear and the crankshaft (a flywheel is alsoprovided between the ring gear and the crack shaft in the crankingrotational force transmission mechanism described in JP-A-2000-274337).When the starter motor cranks the internal combustion engine, therotational force of the starter motor is transmitted to the crankshaftvia the one-way clutch. Then, in response to the crankshaft starting torotate on the force output from the internal combustion engine, theone-way clutch is released and, hence, the rotational force of thecrankshaft is not transmitted to the ring gear side.

In such a cranking rotational force transmission mechanism, lubricantneeds to be supplied to the one-way clutch and other rotationalcomponents to lubricate and cool them. When the internal combustionengine is running, the components on the crankshaft side, such as theflywheel, rotate, however the ring gear that is connected to theinternal combustion engine via the one-way clutch does not rotate.Therefore, the lubricant tends to stagnate in the ring gear side.

The more the lubricant stagnates in the ring gear side, the higher thepossibility of the surface of the stagnant lubricant being stronglyagitated by the rotating one-way clutch and flywheel which are locatedadjacent to the ring gear. Such agitation of the lubricant surfaceincreases the likelihood of bubbling of the lubricant and production ofsludge.

If the lubricant is bubbled and sludge is produced as mentioned above,the components, such as the one-way clutch and some bearings, may bedamaged due to inappropriate lubrication and cooling by the lubricant,and the sealing performances of some sealing members may deteriorate dueto the sludge being caught at them.

SUMMARY OF THE INVENTION

The invention provides a lubrication structure that prevents bubbling oflubricant and production of sludge due to agitation of the surface oflubricant in a cranking rotational force transmission mechanism thattransmits the rotational force of a starter motor in one rotationaldirection to a rotational output shaft of an internal combustion engineand interrupts the transmission of the rotational force of the startermotor in the other rotational direction.

A first aspect of the invention relates to a lubrication structure of acranking rotational force transmission mechanism for an internalcombustion engine, the cranking rotational force transmission mechanismhaving: an outer race member that is coupled with a rotational outputshaft of the internal combustion engine; a ring gear to which arotational drive force is transmitted from a starter motor and a portionof which faces the outer race member from the side of the internalcombustion engine; and a one-way clutch that is provided in a positionacross which the portion of the ring gear faces the outer race memberand that transmits a rotational force of the starter motor in onerotational direction from the ring gear to the outer race member andinterrupts transmission of a rotational force of the starter motor inthe other rotational direction from the ring gear to the outer racemember. The lubrication structure is characterized in that: a sealingmember is provided which oil-seals a gap between the ring gear and theouter race member so that the one-way clutch is in an oil-sealed area inthe internal combustion engine side; and a lubricant-return hole isformed in a portion of the ring gear between a portion of the ring gearat which the one-way clutch is provided and a portion of the ring gearat which the sealing member is provided, the lubricant-return holepenetrating the ring gear from one side to the other side.

In this structure, the lubricant-return hole is formed in the portion ofthe ring gear between where the one-way clutch is provided and where thesealing member is provided. Therefore, even when the ring gear hasstopped rotating in response to the starter motor being turned off aftercompletion of the start of the internal combustion engine, the lubricantstagnating between the ring gear and the outer race member that faceeach other above the sealing member is discharged to the internalcombustion engine side through the lubricant-return hole. As such, anincrease in the amount of the lubricant that stagnates between the ringgear and the outer race member is suppressed and thus the level of thestagnant lubricant surface can be kept low so that the lubricant surfacedoes not reach the outer race member or so that the outer race member isnot soaked deeply into the lubricant even if the lubricant surfacereaches the outer race member. As such, bubbling of the lubricant andproduction of sludge, which may otherwise be caused by the lubricantsurface being agitated by the rotating outer race member, can beprevented.

The lubricant structure according to the first aspect of the inventionmay be such that the lubricant-return hole is provided in plurality andthe lubricant-return holes are formed around a rotational axis of thering gear, such that, at any rotational phase of the ring gear, at leasta part of one of the lubricant-return holes is present below ahorizontal line that is drawn tangent to a lower side of an outerperiphery of the outer race member.

By arranging the position of the lubricant-return holes as describedabove, the level of the surface of the lubricant that stagnates abovethe sealing member after the start of the internal combustion engine canbe adjusted. Thus, the amount of lubricant that stagnates during theoperation of the internal combustion engine can be constantly adjustedto an appropriate amount in a simple manner, and therefore bubbling ofthe lubricant and sludge production can be effectively prevented.

The lubricant structure according to the first aspect of the inventionmay be such that the lubricant-return hole is provided in plurality andthe lubricant-return holes are formed around a rotational axis of thering gear, such that, at any rotational phase of the ring gear, at leasta part of one of the lubricant-return holes is present below ahorizontal line that is drawn tangent to a lower side of an outerperiphery of an outer race of the one-way clutch.

It is considered that the lubricant surface is strongly agitated by, inparticular, the structural elements located on the radially inner sideof the outer race of the one-way clutch. Thus, by arranging thepositions of the lubricant-return holes as described above, the surfaceof the lubricant that stagnates above the sealing member after the startof the internal combustion engine can be adjusted and therefore bubblingof the lubricant and sludge production can be effectively prevented.

The lubricant structure according to the first aspect of the inventionmay be such that an outermost peripheral portion of the outer racemember forms the outer race of the one-way clutch and an innerperipheral end of each of the lubricant-return holes is radially closeto the outer race of the one-way clutch.

By arranging the inner peripheral end of each lubricant-return hole tobe close to the outer race of the one way clutch as described above, theradial dimension of each lubricant return-hole can be made large enoughto quickly discharge the lubricant that has been supplied to between thering gear and the outer race member, which face each other above thesealing member. Thus, an increase in the amount of lubricant thatstagnates above the sealing member can be suppressed.

Further, because the inner peripheral end of each lubricant-return holeis located close to the outer race, the sectional area of the lubricantpassage leading to the lubricant-return holes is relatively small. Thus,a large amount of lubricant, under no circumstances, rapidly flows outfrom the one-way clutch side to the sealing member side. Therefore, itis possible to secure the lubricant of an amount needed for the one-wayclutch and reduce the amount of lubricant that stagnates above thesealing member, so that bubbling of lubricant and production of sludgeare prevented.

The lubricant structure according to the first aspect of the inventionmay be such that at least one of the position of each of thelubricant-return holes, the number of the lubricant-return holes, andthe shape of each of the lubricant-return holes is set such that, whenthe internal combustion engine is running in a steady manner, the levelof a surface of lubricant is below a level that is substantially alignedwith a horizontal line that is drawn tangent to a lower side of theouter periphery of the outer race member.

By adjusting the level of the lubricant surface below the level that issubstantially aligned with the horizontal line as described above,bubbling of lubricant and production of sludge can be sufficientlyprevented.

The lubricant structure according to the first aspect of the inventionmay be such that the sealing member is slidably in contact with theouter periphery of the outer race of the outer race member from theradially outer side of the outer race and at least one of the positionof each of the lubricant-return holes, the number of thelubricant-return holes, and the shape of each of the lubricant-returnholes is set such that, the level of a surface of lubricant is, at leastwhen the internal combustion engine is running in a steady manner,substantially aligned with a horizontal line that is drawn tangent tothe lower side of a slide-contact portion between the outer race and thesealing member.

Because the level of the lubricant surface is substantially aligned withthe horizontal line tangent to the lower side of the slide-contactportion between the outer race and the sealing member, it is possible tosupply a sufficient amount of lubricant to the slide-contact portionbetween the outer race and the sealing member and thus lubricate andcool the sealing member adequately, while preventing bubbling of thelubricant and production of sludge which may otherwise be caused byagitation of the lubricant surface.

The lubricant structure according to the first aspect of the inventionmay be such that the lubricant-return hole is provided in plurality andthe lubricant-return holes are formed such that any two of thelubricant-return holes are not located in phase positions, respectively,which are point-symmetrical to each other about the center of the ringgear.

When multiple lubricant-return holes are provided, a desired flexuralstrength of the entire part of the ring gear can be obtained byarranging the lubricant-return holes such that any two of thelubricant-return holes are not located in phase positions, respectively,which are point-symmetrical to each other about the center of the ringgear, and this prevents deformation of the ring gear and improves thereliability of the oil-sealing by the sealing member.

The lubricant structure according to the first aspect of the inventionmay be such that the ring gear has a spoke portion which is provided onthe radially outer side of the portion where the sealing member isarranged and in which spokes and openings are alternately provided inthe peripheral direction of the ring gear and the phase positions of thelubricant-return holes are arranged so as not to overlap phase positionsat which the radial dimensions of the openings are maximum.

When a spoke portion is provided on the radially outer side of a portionof the ring gear at which the sealing member is provided, for the sakeof, for example, reducing the weight and increasing the ease ofassembly, the phase positions of the lubricant-return holes are arrangedso as not to overlap phase positions at each of which the radialdimension of the opening between the spokes is maximum. As such, adesired flexural strength of the entire part of the ring gear can beobtained, which prevents deformation of the ring gear and improves thereliability of oil-sealing by the sealing member.

The lubricant structure according to the first aspect of the inventionmay be such that a sectional area A1 of a lubricant passage upstream ofthe one-way clutch, a sectional area A2 of a lubricant passagedownstream of the one-way clutch, and a sectional area A3 of a lubricantpassage in the lubricant-return hole are set such that A1≦A2≦A3 is trueat any rotational phase of the ring gear.

With this arrangement, the relation among the flow rates V1, V2, V3 atthe lubricant passages having the sectional areas A1, A2, A3,respectively, is V1≦V2≦V3 in a wide operation range of the internalcombustion engine. As a result, an appropriate amount of lubricant canbe supplied to the one-way clutch, and therefore the lubricant does notstagnate more than necessary in the space in which the one-way clutch isdisposed. Thus, bubbling of lubricant and production of sludge, whichmay otherwise be caused by agitation of the lubricant surface in thisspace, can be prevented. Further, in the above structure, the amount oflubricant to be supplied to between the ring gear and the outer racemember is suppressed, which also contributes to preventing bubbling oflubricant and production of sludge.

A second aspect of the invention relates to a lubrication structure of acranking rotational force transmission mechanism for an internalcombustion engine, the cranking rotational force transmission mechanismhaving: an outer race member that is coupled with a rotational outputshaft of the internal combustion engine; a ring gear to which arotational drive force is transmitted from a starter motor and a portionof which faces the outer race member from the side of the internalcombustion engine; a one-way clutch that is provided in a positionacross which the portion of the ring gear faces the outer race memberand that transmits a rotational force of the starter motor in onerotational direction from the ring gear to the outer race member andinterrupts transmission of a rotational force of the starter motor inthe other rotational direction from the ring gear to the outer racemember, and a sealing member that oil-seals a gap between the ring gearand the outer race member so that the one-way clutch is in an oil-sealedarea in the internal combustion engine side. In this lubricationstructure, a lubricant-return hole is formed in a portion of the ringgear between a portion of the ring gear at which the one-way clutch isprovided and a portion of the ring gear at which the sealing member isprovided, the lubricant-return hole penetrating the ring gear from oneside to the other side, and a wall is provided on the side of an oil panof the internal combustion engine. Formed in this wall is a lubricantoutlet hole through which lubricant that has been returned from thelubricant-return hole falls to the side of a lubricant storage of theoil pan. The lubricant outlet hole is shaped and positioned such that,when the internal combustion engine is running in a steady manner, thelevel of a surface of lubricant stagnating between the ring gear and theouter race member does not exceed a horizontal line that is drawntangent to the lower side of the outer periphery of the outer racemember.

In a case where a wall haying, a lubricant outlet hole is provided toprevent reverse flow of the lubricant from the oil pan side, bypositioning and shaping the lubricant outlet hole as described above, itis possible to appropriately control the level of the surface of thelubricant stagnating between the ring gear and the outer race memberwhile preventing reverse flow of the lubricant from the oil pan side.Thus, the lubricant surface can be prevented from being agitated by theouter race member, which prevents bubbling of lubricant and productionof sludge.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of theinvention will become apparent from the following description ofpreferred embodiments with reference to the accompanying drawings,wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a longitudinal cross-sectional view showing a crankingrotational force transmission mechanism for an internal combustionengine for a vehicle according to the first exemplary embodiment;

FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1;

FIG. 3A to FIG. 3C are views showing the structure of the ring gear usedin the cranking rotational force transmission mechanism of the firstexemplary embodiment;

FIG. 4A to FIG. 4C are views showing the structure of the ring gear usedin the cranking rotational force transmission mechanism of the firstexemplary embodiment;

FIG. 5A to FIG. 5E are views showing the structure of the outer racemember used in the cranking rotational force transmission mechanism ofthe first exemplary embodiment;

FIG. 6 is a longitudinal cross-sectional view illustrating how thelubricant flows in the cranking rotational force transmission mechanismof the first exemplary embodiment;

FIG. 7 is a view showing a state in which the ring gear is stopped at adifferent rotational phase in the cranking rotational force transmissionmechanism of the first exemplary embodiment;

FIG. 8 is a view showing a state in which the ring gear is stopped at adifferent rotational phase in the cranking rotational force transmissionmechanism of the first exemplary embodiment; and

FIG. 9 is a longitudinal cross-sectional view illustrating how thelubricant flows in the cranking rotational force transmission mechanismof the third exemplary embodiment.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 is a longitudinal sectional view showing a cranking rotationalforce transmission mechanism of the first exemplary embodiment that isprovided near a rear end 2 a of a crankshaft 2 of an internal combustionengine for a vehicle. The rear end 2 a of the crankshaft 2 is the end ofthe crankshaft 2 on the side from which the rotational drive power ofthe internal combustion engine is output to a clutch or a torqueconverter. FIG. 2 is a cross-sectional view of the crankshaft 2 that istaken along the perpendicular plane II-II indicated in FIG. 1, as seenfrom the left side of FIG. 1, i.e., from the front side of the internalcombustion engine.

A journal bearing is formed by a cylinder block 4 and a ladder beam 6.The crankshaft 2 is rotatably supported by the cylinder block 4 via ajournal 2 b. Thus, the crankshaft 2 is arranged such that the rear end 2a of the crankshaft 2 protrudes from the lower rear portion of thecylinder block 4. A ring gear 12 is provided on the peripheral surfaceof a large-diameter portion 8 formed at the rear end 2 a of thecrankshaft 2 with a rolling bearing, which is a ball bearing 10 in thisexemplary embodiment, being interposed therebetween. An outer racemember 14 and a flywheel (or a drive plate) 16 are fixed on the rearsurface of the large-diameter portion 8 of the crankshaft 2 using bolts18. Thus, the outer race member 14 and the flywheel 16 rotate togetherwith the crankshaft 2.

As shown in FIG. 3A to FIG. 3C and FIG. 4A to FIG. 4C, the ring gear 12is a circular disk having a large opening at the center thereof and abending portion 12 a that radially bends at a right angle along theentire periphery. FIG. 3A is a perspective view showing the front sideof the ring gear 12, FIG. 3B is a front view of the ring gear 12, FIG.3C is a right side view of the ring gear 12, FIG. 4A is a perspectiveview showing the rear side of the ring gear 12, FIG. 4B is a rear viewof the ring gear 12, and FIG. 4C is a left side view of the ring gear12.

An inner race 22 for a one-way clutch 20 is formed like a flange at theinner peripheral edge of the ring gear 12. A gear portion 12 b is formedlike a ring at the outer periphery of the ring gear 12. The ring-gear 12is disposed, via the ball bearing 10 described above, on the outerperiphery of the large-diameter portion 8 of the crankshaft 2 so as tobe located on the opposite side of the one-way clutch 20, i.e., so as tobe closer to the center side (i.e., the axis C side) than the one-wayclutch 20 is. The ball bearing 10 is attached onto the outer peripheryof the large-diameter portion 8 of the crankshaft 2 by press-fitting,and the inner race 22 is fitted to the outer periphery of the ballbearing 10. As such, when the one-way clutch 20 is in a released state,because of the ball bearing 10, the ring gear 12 can freely rotateindependently of the rotation of the crankshaft 2.

Referring to FIG. 2, the gear portion 12 b of the ring gear 12 isconstantly in mesh with a pinion gear 28 that is provided at a positionlower than the crankshaft 2 and is rotated by the a starter motor 26.That is, when cranking the internal combustion engine, the ring gear 12is rotated by the rotational force of the starter motor 26 transmittedvia the pinion gear 28.

Lubricant-return holes 12 d are formed at an intermediate portion 12Cbetween the inner race 22 and the bending portion 12 a, which appearslike a flat ring, so as to be located in a circle around the axis C atthe same phase intervals. Each lubricant-return hole 12 d penetrates theintermediate portion 12 c from one side to the other side. Note that theintermediate portion 12 c can be regarded as “a portion of the ring gearbetween a portion of the ring gear at which the one-way clutch isprovided and a portion of the ring gear at which the sealing member isprovided” as recited in the invention. In this exemplary embodiment, thenumber of the lubricant-return holes 12 d is seven, and all thelubricant-return holes 12 d are formed in the same shape. As shown inFIG. 3B, the lubricant-return holes 12 d are shaped and positioned suchthat any lubricant-return hole 12 d is not present in the region of theintermediate portion 12 c that is point-symmetrical about the axis C toa phase region a occupied by other lubricant-return hole 12 d.

Seven openings 12 f are formed at an outer periphery portion 12 e of thering gear 12, which connects the bending portion 12 a and the gearportion 12 b. Between any two adjacent openings 12 f is formed a spoke12 g. Having such a spoke portion comprised of the openings 12 f and thespokes 12 g, the ring gear 12 is made light in weight. The relativephase positions of the openings 12 f and the lubricant-return holes 12 dare such that each lubricant-return hole 12 d and each spoke 12 g arelocated in line with each other in the radial direction of the ring gear12 and the width of each lubricant-return hole 12 d is smaller than thewidth of the radially inner end of each spoke 12 g, as shown in FIG. 3B.Thus, the phase position of each lubricant-return hole 12 d and thephase position of the maximum width region M of each opening 12 f,across which the dimension D of the opening 12 f in the radial directionof the ring gear 12 is maximum, do not overlap with each other.

Referring to FIG. 5A to FIG. 5E, the outer race member 14 is a circulardisk having an opening 14 a formed at the center thereof and an outerrace 30 for the one-way clutch 20 which is formed at the outermostperiphery so as to protrude perpendicularly therefrom. Multiple boltholes 14 b, which are through holes into which bolts are inserted whenbolting the outer race member 14 to the large-diameter portion 8 of thecrankshaft 2, are formed in a circle around the opening 14 a. In thisexemplary embodiment, the number of the bolt holes 14 b is eight. FIG.5A is a front view of the outer race member 14, FIG. 5B is a rear viewof the outer race member 14, FIG. 5C is a right side view of the outerrace member 14, FIG. 5D is a perspective view showing the front side ofthe outer race member 14, and FIG. 5E is a perspective view showing therear side of the outer race member 14.

That is, being attached to the side of the large-diameter portion 8 ofthe crankshaft 2 as described above, the outer race member 14 isarranged relative to the ring gear 12 such that the outer race 30radially faces the inner race 22 of the ring gear 12, as shown inFIG. 1. During assembly, a cage 20 b having a sprag 20 a is put on theinner race 22 of the ring gear 12 when or before putting the outer race30 in position, so that the sprag 20 a is sandwiched between the innerrace 22 and the outer race 30. This is how the one-way clutch 20 isassembled.

Being structured as describe above, the one-way clutch 20 engages theouter race member 14 and the ring gear 12 when the ring gear 12 isrotating in the direction to transmit the torque from the starter motor26 to the outer race member 14 (the clockwise direction indicated by thearrow in FIG. 2), thus enabling the crankshaft 2 to be rotated by thestarter motor 26. Then, when the rotation speed of the outer race member14 rotating together with the crankshaft 2 exceeds the rotation speed ofthe ring gear 12 being rotated by the starter motor 26 after theinternal combustion engine starts running on its own, the rotation ofthe ring gear 12 relative to the outer race member 14 becomes reverserotation. Thus, the one-way clutch 20 is released. As such, although thepinion gear 28 and the ring gear 12 are constantly in mesh with eachother, the starter motor 26 may be turned off after starting theinternal combustion engine.

In order to lubricate the ball bearing 10 and the one-way clutch 20,lubricant Oj is supplied through a lubricant passage 4 a in the cylinderblock 4, as indicated by the arrow in FIG. 1, and the lubricant Oj isthen injected towards the ball bearing 10. Also, lubricant Or issupplied to the slide surfaces of the journal 2 b of the crankshaft 2via lubricant passages in the cylinder block 4 and the crankshaft 2.Part of the lubricant Or flows to the ball bearing 10 side. Thelubricants Oj, Or that have flown to the ball bearing 10 then passthrough the inside of the ball bearing 10 and flow into the one-wayclutch 20.

A first oil sealing member 32 and a second oil sealing member 34, whichare ring-shaped sealing members, are provided to prevent the lubricantsOj, Or from leaking to the outside. The first oil sealing member 32 isinterposed between the outer race 30 of the outer race member 14 and thebending portion 12 a of the ring gear 12. The first oil sealing member32 is fixed to the ring gear 12 by being fitted to the internalperipheral side of the bending portion 12 a, so that a seal rip 32 athat is formed at the internal periphery of the first oil sealing member32 is slidably in contact with a sealed slide surface 30 a that is theouter peripheral surface of the outer race 30, forming a slide-contactportion. Being thus arranged, the first oil sealing member 32 oil-sealsbetween the outer race member 14 and the ring gear 12.

The second oil sealing member 34 is provided on the side of the bendingportion 12 a that is opposite to where the first oil sealing member 32is provided (i.e., the radially outer side of the first oil sealingmember 32). The second oil sealing member 34 is fixed at the positionshown in the drawings by the portion of the second oil sealing member 34above the crankshaft 2 being fitted mainly to the internal peripheralside of an arc-shaped seal fitting portion 4 b of the cylinder block 4and by the portion of the second oil sealing member 34 below thecrankshaft 2 being fitted mainly to the internal peripheral side of anarc-shaped seal fitting portion 36 a that is provided at the rear end ofan oil pan 36, so that the seal rip 32 a formed at the internalperiphery of the second oil sealing member 34 is slidably in contactwith the outer peripheral surface of the bending portion 12 a. Beingthus arranged, the second oil sealing member 34 oil-seals between thering gear 12 and the internal combustion engine (i.e., the cylinderblock 4 and the oil pan 36).

As mentioned above, when cranking the internal combustion engine, thestarter motor 26 rotates the ring gear 12, and after the internalcombustion engine starts running on its own, the ring gear 12 stopsrotating due to the slippage at the one-way clutch 20. When the internalcombustion engine is running, the lubricants Oj, Or, as described above,pass through the inside of the ball bearing 10 and reach the rotatingouter race member 14. Then, the lubricants Oj, Or further proceedthrough the gap between an outer race 10 a of the ball bearing 10 andthe outer race member 14 and the gap between the inner race 22 and theouter race member 14, as indicated by the arrow F1 in FIG. 6, and thenthe lubricants Oj, Or flow into a space 21 in which the one-way clutch20 is disposed, lubricating the one-way clutch 20.

After lubricating the one-way clutch 20, the lubricants Oj, Or proceedthrough the gap between the outer race 30 and the intermediate portion12 c of the ring gear 12, as indicated by the arrow F2 in FIG. 6, andthen flow into a space 12 h that is provided above the bending portion12 a and in which the first oil sealing member 32 is disposed. The space12 f is oil-sealed from the outside by the first oil sealing member 32fixed on the bending portion 12 a. When the ring gear 12 is notrotating, the lubricant stagnates in a stagnation region OS in the space12 h above the bending portion 12 a of the ring gear 12, which ishatched by the horizontal broken lines in FIG. 2 and FIG. 6.

However, in the intermediate portion 12 c of the ring gear 12 whichforms a wall of the stagnation region OS, the lubricant-return holes 12d are formed so as to penetrate the intermediate portion 12 c from oneside to the other side thereof, as described above. Therefore, in thestate where the ring gear 12 is stopped at the rotational phase positionindicated in FIG. 2, one of the lubricant-return holes 12 d is locatedat the lowest position, and the lubricant in the space 12 f returns tothe oil pan 36 side via that lubricant-return hole 12 d, as indicated bythe arrow F3 in FIG. 6. When the internal combustion engine is runningin a steady manner, due to the lubricant-return hole 12 d at the lowestposition, the level of the lubricant surface Of in the stagnation regionOS remains substantially constant at a level slightly below a lowestposition level RL of the sealed slide surface 30 a of the outer race 30(i.e., the horizontal line that is tangent to the lower side of theouter periphery of the outer race member 14).

FIG. 7 and FIG. 8 each show an example in which the ring gear 12 isstopped at a different rotational phase position. As is evident fromFIG. 2, FIG. 7, and FIG. 8, the positions, the number, and the shapes ofthe lubricant-return holes 12 d are determined such that at least a partof one of the lubricant-return hole 12 d is present below the lowestposition level RL at any rotational phase of the ring gear 12.

In the example shown in FIG. 7, the level of the lubricant surface Of inthe stagnation region OS depends upon the state of the singlelubricant-return hole 12 d that is present at the lowest position, as inthe example shown in FIG. 2. However, because the position of thatlubricant-return hole 12 d, through which the lubricant returns to theoil pan 36 side, is slightly above the position of the lowestlubricant-return hole 12 d in the example shown in FIG. 2. Thus, whenthe internal combustion engine is running in a steady manner, thelubricant surface Of remains substantially constant at a positionslightly above the position at which the lubricant surface Of remainssubstantially constant in the example shown in FIG. 2. Note that, in theexample shown in FIG. 7, the lubricant surface is at the same level asthe lowest position level RL.

In the example shown in FIG. 8, two lubricant-return holes 12 d arepresent at the lowest position, and therefore the lubricant surface Ofin the stagnation region OS depends upon the states of these twolubricant-return holes 12 d. Because each of the two lubricant-returnholes 12 d is located slightly above the position of the lowestlubricant-return hole 12 d in the example shown in FIG. 7, the lubricantsurface Of, when the internal combustion engine is running in a steadymanner, remains substantially constant at a position slightly above theposition at which the lubricant surface Of remains substantiallyconstant in the example shown in FIG. 7.

As described above, the area of the portion of the lubricant-return hole12 d that is present below the lowest position level RL, when theinternal combustion engine is running in a steady manner, is determinedso that the level of the lubricant surface Of substantially matches thelowest position level RL. As such, when the internal combustion engineis running, in particular, when the internal combustion engine isrunning in a steady manner, any portion of the outer race 30 is notsoaked deeply into the lubricant. That is, the lubricant is not heavilyagitated by the components or portions that are rotating on the radiallyinner side with respect to the outer race 30. Meanwhile, theslide-contact portion between the seal rip 32 a of the first oil sealingmember 32 and the sealed slide surface 30 a of the outer race 30 islocated in the position which the lubricant is brought into contact withas the lubricant surface Of waves, even in the state where the level ofthe lubricant surface Of is as low as shown in FIG. 2.

When the level of the lubricant surface Of slightly goes down in theexample shown in FIG. 2, the lubricant may not be brought into contactwith the slide-contact portion even when the lubricant surface Of iswaving. However, because an inner peripheral end 12 i of eachlubricant-return hole 12 d is located near the outer race 30 as shown inFIG. 2, the lubricant flows through the narrow gap between the outerrace 30 and the intermediate portion 12 c of the ring gear 12, asindicated by the arrow F2 in FIG. 6, and the lubricant is then splashedinto the space 12 h from the narrow gap. Since the point from which thelubricant is splashed is located at the same level and thus is veryclose to the slide-contact portion, the lubricant reaches theslide-contact portion in the form of droplets.

The positions, the number; and the shapes of the lubricant-return holes12 d, which serve to adjust the level of the lubricant surface Of asdescribed above, are determined such that a flow rate V2 at thelubricant passage indicated by the arrow F2 in FIG. 6 when the internalcombustion engine is running in a steady manner is equal to a flow rateV3 at the lubricant passage indicated by the arrow F3 in FIG. 6 when thelubricant surface Of in the space 12 h is substantially at the lowestposition level RL of the sealed slide surface 30 a. Specifically, forexample, theoretical calculations are hydrodynamically performed basedon the sectional areas of the lubricant passages indicated by the arrowsF2, F3, the rotation speed of the outer race member 14 during the steadyrunning of the internal combustion engine, the flow characteristics ofthe lubricant, and so on. Then, at least one of the position of eachlubricant-return hole 12 d, the number of the lubricant-return holes 12d, the shape of each lubricant-return hole 12 d is determined based onthe actual measurements of a sample for which the results of thetheoretical calculations are taken into consideration. Alternatively,this determination may be made based on the actual measurements from thebeginning. Further, the dimensions of the gap indicated by the arrow F2may also be set.

Meanwhile, as described above, the capacity of returning the lubricantthrough the lubricant-return holes varies depending upon the rotationalphase at which the ring gear 12 is stopped. Therefore, for example, eachlubricant-return hole 12 d may be formed such that a sectional area ofthe lubricant passage in the lubricant-return hole 12 d is obtained whenthe ring gear 12 is stopped at a typical rotational phase, e.g., at therotational phase shown in FIG. 7. Alternatively, each lubricant-returnhole 12 d may be formed such that the lubricant surface Of issubstantially aligned with the lowest position level RL in the statewhere the sectional area of the lubricant passage in thelubricant-return hole 12 d is minimum, such as when the ring gear 12 isat the rotational phase shown in FIG. 8.

The following advantageous effects can be obtained according to thefirst exemplary embodiment described above.

(1) Because the lubricant-return holes 12 d are provided in theintermediate portion 12 c of the ring gear 12, even when the ring gear12 has stopped rotating in response to the starter motor 26 being turnedoff after completion of the start of the internal combustion engine, thelubricant stagnating above the first oil sealing member 32 can be easilyreturned to the internal combustion engine side, i.e., to the oil pan 36side, via the lubricant-return holes 12 d.

As such, the level of the lubricant surface Of the lubricant remainingbetween the ring gear 12 and the outer race member 14 does not rise.Therefore, the lubricant surface Of does not reach the outer race 30 andthe one-way clutch 20 which are rotating while the internal combustionis running, and even if the lubricant surface Of reaches them, theone-way clutch 20 and the outer race 30 are not deeply soaked into thelubricant. As such, bubbling of lubricant and production of sludge,which may otherwise be caused by agitation of the lubricant surface Of,can be prevented.

(2) At least a part of one of the lubricant-return holes 12 d is presentbelow the lowest position level RL, which corresponds to the lower sideof the outer periphery of the outer race member 14 (i.e., the sealedslide surface 30 a), at any rotational phase of the ring gear 12.Therefore, the lubricant surface Of that has been present above thefirst oil sealing member 32 is appropriately adjusted by thelubricant-return holes 12 d after the start of the internal combustionengine. Thus, bubbling of lubricant and production of sludge can beprevented.

Further, because the lubricant-return holes 12 d are set such that thelevel of the lubricant surface Of is substantially aligned with thelowest position level RL, the amount of lubricant that stagnates abovethe first oil sealing member 32 can be effectively reduced. Thus,bubbling of lubricant and production of sludge, which may otherwise becaused by agitation of the lubricant surface Of, can be prevented, andthe portion between the outer race 30 and the first oil sealing member32 can be sufficiently cooled and lubricated.

(3) The inner peripheral end 12 i of each lubricant-return hole 12 d isradially located near the outer race 30. Therefore, the radial dimensionof each lubricant-return hole 12 d can be made large enough to quicklydischarge the lubricant that has been supplied to the space 21 in whichthe one-way clutch 20 is disposed and which is located above the firstoil sealing member 32. As such, ah increase in the amount of lubricantthat stagnates above the first oil sealing member 32 can be suppressed.

Further, because the gap between the ring gear 12 and the outer racemember 14 is narrow at the internal peripheral end 12 i of the ring gear12, under no circumstance, a large amount of lubricant rapidly flows outfrom the space 21 in which the one-way clutch 20 is disposed. Therefore,it is possible to secure the lubricant of an amount needed for theone-way clutch 20 and reduce the amount of lubricant that stagnatesabove the first sealing member 32. Thus, bubbling of lubricant andproduction of sludge, which may otherwise be caused by agitation of thelubricant surface Of, can be prevented.

(4) The lubricant-return holes 12 d are arranged such that anylubricant-return hole 12 d is not located at a position that ispoint-symmetrical to other lubricant-return hole 12 d about the centerof the ring gear 12 (the axis C). With this arrangement, the necessaryflexural strength of the entire part of the ring gear 12 can be obtainedwithout increasing the rigidity of the ring gear 12 by, for example,increasing the thickness of the ring gear 12, and therefore the ringgear 12 can be made light in weight.(5) As described above, the spoke portion is provided in the radiallyouter side of the first oil sealing member 32 for the sake of reducingthe weight and increasing the ease of assembly, and the phase positionof each lubricant-return hole 12 d is set so as not to overlap the phaseposition at which the radial dimension of the opening 12 f is maximum(maximum width region M). According to this structure, a desiredflexural strength of the entire part of the ring gear 12 can beobtained, which prevents deformation of the ring gear 12 and therebyimproves the reliability of oil-sealing by the first oil sealing member32 and the second oil sealing member 34. As such, the rigidity of thering gear 12 can be made enough to prevent low-frequency resonance whenthe internal combustion engine is running in a steady manner.

In the second exemplary embodiment, the relation between the sectionalarea A1 of the lubricant passage that is indicated by the arrow F1 inFIG. 6 and is located upstream of the one-way clutch 20 and thesectional area A2 of the lubricant passage that is indicated by thearrow F2 in FIG. 6 and is located downstream of the one-way clutch 20 isA1≦A2. Thus, the relation between the flow rate V1 at the lubricantpassage indicated by the arrow F1 and the flow rate V2 at the lubricantpassage indicated by the arrow F2 can be maintained to be V1≦V2 in awide operation range of the internal combustion engine.

Further, the relation between the sectional area A2 of the lubricantpassage downstream of the one-way clutch 20 and the sectional area A3 ofthe lubricant passage in the lubricant-return hole 12 d, which isindicated by the arrow F3 in FIG. 6, is A2≦A3 at any rotational phase ofthe ring gear 12. Thus, the relation between the flow rate V2 at thelubricant passage indicated by the arrow F2 and the flow rate V3 at thelubricant passage indicated by the arrow F3 can be maintained to beV2≦V3 in a wide operation range of the internal combustion engine.

That is, the relation between the sectional areas of the lubricantpassages in the respective gaps and lubricant-return holes 12 d is setto be A1≦A2≦A3, so that the relation between the flow rates V1, V2, V3can be maintained to be V1≦V2≦V3 in a wide operation range of theinternal combustion engine.

Other structures in the second exemplary embodiment are the same asthose in the first exemplary embodiment. According to the secondexemplary embodiment described above, the following advantageous effectscan be obtained.

(1) Because the relation among the flow rates V1, V2, V3 can bemaintained to be V1≦V2≦V3 in a wide operation range of the internalcombustion engine, an appropriate amount of lubricant can be supplied tothe one-way clutch 20, and therefore the lubricant does not stagnate inthe space 21. Thus, bubbling of lubricant and production of sludge,which may otherwise be caused by agitation of the lubricant surface inthe space 21, can be prevented.

Further, in the structure described above, the lubricant does notstagnate in the space 12 h above the first oil sealing member 32, andtherefore bubbling of lubricant and production of sludge, which mayotherwise be caused by agitation of the lubricant surface in the space12 h, can be prevented. Note that, as mentioned in connection with thefirst exemplary embodiment, the lubricant is splashed into the space 12h from the narrow gap between the outer race 30 and the intermediateportion 12 c of the ring gear 12, which is indicated by the arrow F2 inFIG. 6. Therefore, the lubricant reaches, in the form of droplets, theslide-contact portion between the seal rip 32 a of the first oil sealingmember 32 and the sealed slide surface 30 a of the outer race 30, sothat the slide-contact portion is lubricated and cooled.

In the third exemplary embodiment, as shown in FIG. 9, the sectionalarea of each lubricant-return hole 112 d of the ring gear 112 is solarge that the lubricant-return hole 112 d does not serve to adjust thelevel of the lubricant surface Of in the space 122 h above the first oilsealing member 132. Instead, the adjustment of the level of thelubricant surface Of is accomplished by the position (i.e., thesectional area A1) and shape (i.e., the height H) of a lubricant outlethole 136 c that is formed in a wall 136 b of the oil pan 136 so as toface the lubricant-return hole 112 d.

Specifically, the lubricant outlet hole 136 c is positioned and shapedsuch that a flow rate V14 at the lubricant passage indicated by thearrow F14 in FIG. 9 and a flow rate V15 at the lubricant passageindicated by the arrow F15 in FIG. 9 are equal with each other in thestate where the level of the lubricant surface Of is lower than thesealed slide surface 130 a of the outer race 130 when the internalcombustion engine is running in a steady manner. That is, the lubricantoutlet hole 136 c is positioned and shaped such that, when the internalcombustion engine is running in a steady state, the level of thelubricant surface Of lubricant stagnating between the ring gear 112 andthe outer race member 114 does not exceed the horizontal line that istangent to the lower side of the sealed slide surface 130 a of the outerrace 130. Note that FIG. 9 shows the state in which the lubricantsurface Of and the horizontal line are at the same level. Also, notethat the flow rate V14 at the lubricant passage indicated by the arrowF14 is equal to the sum of the flow rate V12 at the lubricant passageindicated by the arrow F12 (or F11) and the flow rate V13 at thelubricant passage indicated by the arrow F13.

For example, the sectional area A15 of the lubricant outlet hole 136 cand the height H from the lubricant outlet hole 136 c to the sealedslide surface 130 a are set such that the relation among the flow rateV14, the sectional area A15, the height H, and the flow rate coefficientCf of the lubricant satisfies Inequality (1) shown below.

V14≦Cf·A15√{square root over ( )}(2gH)  (1)

where √{square root over ( )}( ) is an operator indicating the squareroot of the value in the parentheses and g represents the gravitationalacceleration. The right side of Inequality (1) represents the amount oflubricant that is discharged via the lubricant outlet hole 136 c perunit time; i.e., the flow rate V15.

Other structures in the third exemplary embodiment are the same as thosein the first exemplary embodiment. According to the third exemplaryembodiment, the following advantageous effects can be obtained.

(1) By using the lubricant outlet hole 136 in place of thelubricant-return holes 112 d, it is possible to control the level of thelubricant surface Of the lubricant stagnating between the ring gear 112and the outer race member 114 while preventing reverse flow of thelubricant from the oil pan 136 side. Thus, the lubricant surface Of canbe prevented from being agitated by the outer race member 114, andtherefore prevents bubbling of lubricant and production of sludge can beprevented.

Further, in the structure of the third exemplary embodiment, even whenthe level of the lubricant surface Of is set below the foregoinghorizontal line, because the lubricant is splashed from the narrow gapbetween the outer race 130 and the intermediate portion 112 c of thering gear 112, which is indicated by the arrow F12 in FIG. 9, thelubricant reaches, in the form of droplets, the slide-contact portionbetween the seal rip 132 a of the first oil sealing member 132 and thesealed slide surface 130 a of the outer race 130, so that theslide-contact portion is lubricated and cooled.

The present invention is not limited to the embodiments described abovebut may be modified into the following alternative embodiments.

While the lubricant-return holes are provided at the same phaseintervals in each of the exemplary embodiments described above, thelubricant-return holes may not be provided at the same phase intervals.That is, even if the lubricant-return holes are not provided at the samephase intervals, the advantageous effect (4) that has been descriedabove in connection with the first exemplary embodiment can be obtainedby arranging the positions of the respective lubricant-return holes suchthat any lubricant-return hole is not located at a position that ispoint-symmetrical to another lubricant-return hole about the center ofthe ring gear. That is, forming an odd number of lubricant-return holesat the same phase intervals in the intermediate portion of the ring gearprovides an arrangement of the lubricant-return holes in which anylubricant-return hole is not located at a position that ispoint-symmetrical to another lubricant-return hole about the center ofthe ring gear. Such an arrangement of the lubricant-return holes can beobtained also by forming an even number of lubricant-return holes atuneven intervals such that any lubricant-return hole is not located at aposition that is point-symmetrical to another lubricant-return holeabout the center of the ring gear.

The same applies to the relation between the lubricant-return holes andthe openings formed in the spoke portion. That is, even when thelubricant-return holes are provided at uneven intervals, theadvantageous effect (5) that has been described above in connection withthe first exemplary embodiment can be obtained by arranging thepositions of the lubricant-return holes and the positions of theopenings of the spoke portion such that the phase position of eachlubricant-return hole does not overlap the phase position of the portionof each opening in the spoke portion at which the radial dimension ofthe same opening is maximum.

While the level of the lubricant surface Of is substantially alignedwith the horizontal line tangent to the lower side of the sealed slidesurface (i.e., the lowest position level RL) in the first exemplaryembodiment, if lubrication and cooling of the first oil sealing memberare not directly performed by the lubricant surface Of, the level of thelubricant surface Of may be set below the lowest position level RL.

For example, the level of the lubricant surface Of may be set below thelowest position level RL in a structure in which the lubricant issplashed from the narrow gap between the outer race and the intermediateportion of the ring gear (i.e., the gap indicated by the arrow F2, thegap indicated by the arrow F12) so that the lubricant reaches, in theform of droplets, the seal rip of the first oil sealing member and thesealed slide surface of the outer race, as in the second and thirdexemplary embodiments.

While the outer periphery of the outer race member is the outer race ofthe one-way clutch in each of the exemplary embodiments described above,if there is any part or portion that protrudes outward from the outerperiphery of the outer race member, the structures in the respectiveexemplary embodiments may be designed by using the horizontal line thatis tangent to the protruding part or portion as the lowest positionlevel RL. Nevertheless, if the structural elements of the outer racemember that are present on the radially inner side of the outer race aresignificantly responsible for agitation of the lubrication surface Of,the lubricant surface Os may be set with respect to the outer peripheralsurface of the outer race, as it is in the respective exemplaryembodiments described above.

In each of the exemplary embodiments, the outer race member may beprovided as a part of the flywheel (or the drive plate), rather than asan independent member. That is, the flywheel (or the drive plate) andthe outer race member may be provided as a single component by forming aportion of the flywheel (or the drive plate) as an outer race. In thiscase, the flywheel (or the drive plate) corresponds to the outer racemember.

While the invention has been described with reference to the exampleembodiment thereof, it is to be understood that the invention is notlimited to the example embodiment and construction. To the contrary, theinvention is intended to cover various modifications and equivalentarrangements. In addition, while the various elements of the exampleembodiment are shown in various combinations and configurations, whichare exemplary, other combinations and configurations, including more,less or only a single element, are also within the sprit and scope ofthe invention.

1. A lubrication structure of a cranking rotational force transmission mechanism for an internal combustion engine, the cranking rotational force transmission mechanism having an outer race member that is coupled with a rotational output shaft of the internal combustion engine; a ring gear to which a rotational drive force is transmitted from a starter motor and a portion of which faces the outer race member from the side of the internal combustion engine; and a one-way clutch that is provided in a position across which the portion of the ring gear faces the outer race member and that transmits a rotational force of the starter motor in one rotational direction from the ring gear to the outer race member and interrupts transmission of a rotational force of the starter motor in the other rotational direction from the ring gear to the outer race member, the lubrication structure comprising: a sealing member that is provided to oil-seal a gap between the ring gear and the outer race member so that the one-way clutch is in an oil-sealed area in the internal combustion engine side; and a lubricant-return hole that is formed in a portion of the ring gear between a portion of the ring gear at which the one-way clutch is provided and a portion of the ring gear at which the sealing member is provided, the lubricant-return hole penetrating the ring gear from one side to the other side; and wherein the lubricant-return hole is provided in plurality and the lubricant-return holes are formed around a rotational axis of the ring gear, such that, at any rotational phase of the ring gear, at least a part of one of the lubricant-return holes is present below a horizontal line that is drawn tangent to a lower side of an outer periphery of the outer race member.
 2. The lubrication structure according to claim 1, wherein the lubricant-return hole is provided in plurality and the lubricant-return holes are formed around a rotational axis of the ring gear, such that, at any rotational phase of the ring gear, at least a part of one of the lubricant-return holes is present below a horizontal line that is drawn tangent to a lower side of an outer periphery of an outer race of the one-way clutch.
 3. The lubrication structure according to claim 1, wherein an outermost peripheral portion of the outer race member forms an outer race of the one-way clutch, and an inner peripheral end of each of the lubricant-return holes is radially close to the outer race of the one-way clutch.
 4. The lubrication structure according to claim 1, wherein at least one of the position of each of the lubricant-return holes, the number of the lubricant-return holes, and the shape of each of the lubricant-return holes is set such that, when the internal combustion engine is running in a steady manner, the level of a surface of lubricant is below a level that is substantially aligned with a horizontal line that is drawn tangent to a lower side of the outer periphery of the outer race member.
 5. The lubrication structure according to claim 1, wherein the sealing member is slidably in contact with the outer periphery of an outer race of the one-way clutch from the radially outer side of the outer race, the outer race being formed with the outer race member, and at least one of the position of each of the lubricant-return holes, the number of the lubricant-return holes, and the shape of each of the lubricant-return holes is set such that, when the internal combustion engine is running in a steady manner, the level of a surface of lubricant is substantially aligned with a horizontal line that is drawn tangent to the lower side of a slide-contact portion between the outer race and the sealing member.
 6. A lubrication structure of a cranking rotational force transmission mechanism for an internal combustion engine, the cranking rotational force transmission mechanism having an outer race member that is coupled with a rotational output shaft of the internal combustion engine; a ring gear to which a rotational drive force is transmitted from a starter motor and a portion of which faces the outer race member from the side of the internal combustion engine; and a one-way clutch that is provided in a position across which the portion of the ring gear faces the outer race member and that transmits a rotational force of the starter motor in one rotational direction from the ring gear to the outer race member and interrupts transmission of a rotational force of the starter motor in the other rotational direction from the ring gear to the outer race member, the lubrication structure comprising: a sealing member that is provided to oil-seal a gap between the ring gear and the outer race member so that the one-way clutch is in an oil-sealed area in the internal combustion engine side; and a lubricant-return hole that is formed in a portion of the ring gear between a portion of the ring gear at which the one-way clutch is provided and a portion of the ring gear at which the sealing member is provided, the lubricant-return hole penetrating the ring gear from one side to the other side; and wherein the lubricant-return hole is provided in plurality, and the lubricant-return holes are formed such that any two of the lubricant-return holes are not located in phase positions, respectively, which are point-symmetrical to each other about the center of the ring gear.
 7. A lubrication structure of a cranking rotational force transmission mechanism for an internal combustion engine, the cranking rotational force transmission mechanism having an outer race member that is coupled with a rotational output shaft of the internal combustion engine; a ring gear to which a rotational drive force is transmitted from a starter motor and a portion of which faces the outer race member from the side of the internal combustion engine; and a one-way clutch that is provided in a position across which the portion of the ring gear faces the outer race member and that transmits a rotational force of the starter motor in one rotational direction from the ring gear to the outer race member and interrupts transmission of a rotational force of the starter motor in the other rotational direction from the ring gear to the outer race member, the lubrication structure comprising: a sealing member that is provided to oil-seal a gap between the ring gear and the outer race member so that the one-way clutch is in an oil-sealed area in the internal combustion engine side; and a lubricant-return hole that is formed in a portion of the ring gear between a portion of the ring gear at which the one-way clutch is provided and a portion of the ring gear at which the sealing member is provided, the lubricant-return hole penetrating the ring gear from one side to the other side; and wherein the ring gear has a spoke portion which is provided on the radially outer side of the sealing member and in which spokes and openings are alternately provided in the peripheral direction of the ring gear, and the phase positions of the lubricant-return holes are arranged so as not to overlap phase positions at which the radial dimensions of the openings are maximum.
 8. The lubrication structure according to claim 1, wherein a sectional area A1 of a lubricant passage upstream of the one-way clutch, a sectional area A2 of a lubricant passage downstream of the one-way clutch, and a sectional area A3 of a lubricant passage in the lubricant-return hole are set such that A1≦A2≦A3 is true at any rotational phase of the ring gear.
 9. The lubrication structure according to claim 1, further comprising a wall which is provided on the side of an oil pan of the internal combustion engine; the wall having a lubricant outlet hole formed therein, wherein lubricant that has been returned from the lubricant-return hole falls to the side of a lubricant storage of the oil pan through the lubricant outlet hole, wherein the lubricant outlet hole is shaped and positioned such that, when the internal combustion engine is running in a steady manner, the level of a surface of lubricant stagnating between the ring gear and the outer race member does not exceed a horizontal line that is drawn tangent to the lower side of the outer periphery of the outer race member.
 10. The lubrication structure according to claim 9, wherein V14≦Cf·A15√{square root over ( )}(2gH) is true where V14 is a flow rate of lubricant flowing to the lubricant outlet hole, A15 is the sectional area of a lubricant passage in the lubricant outlet hole, H is the height from the lubricant outlet hole to a sealed slide surface of the sealing member at which the sealing member slidably contacts the outer race member, g is a gravitational acceleration, and cf is a flow rate coefficient of lubricant.
 11. The lubrication structure according to claim 6, wherein a sectional area A1 of a lubricant passage upstream of the one-way clutch, a sectional area A2 of a lubricant passage downstream of the one one-way clutch, and a sectional area A3 of a lubricant passage in the lubricant-return hole are set such that A1≦A2≦A3 is true at any rotational phase of the ring gear.
 12. The lubrication structure according to claim 6, further comprising a wall which is provided on the side of an oil pan of the internal combustion engine, the wall having a lubricant outlet hole formed therein, wherein lubricant that has been returned from the lubricant-return hole falls to the side of a lubricant storage of the oil pan through the lubricant outlet hole, wherein the lubricant outlet hole is shaped and positioned such that, when the internal combustion engine is running in a steady manner, the level of a surface of lubricant stagnating between the ring gear and the outer race member does not exceed a horizontal line that is drawn tangent to the lower side of the outer periphery of the outer race member.
 13. The lubrication structure according to claim 12 wherein V14≦Cf·A15√{square root over ( )}(2gH) is true of where V14 is a flow rate of lubricant flowing to the lubricant outlet hole A15 is the sectional area of a lubricant passage in the lubricant outlet hole, H is the height from the lubricant outlet hole to a sealed slide surface of the sealing member at which the sealing member slidably contacts the outer race member, g is a gravitational acceleration and cf is a flow rate coefficient of lubricant.
 14. The lubrication structure according to claim 7, wherein a sectional area A1 of a lubricant passage upstream of the one-way clutch, a sectional area A2 of a lubricant passage downstream of the one-way clutch, and a sectional area A3 of a lubricant passage in the lubricant-return hole are set such that A1≦A2≦A3 is true at any rotational phase of the ring gear.
 15. The lubrication structure according to claim 7, further comprising a wall which is provided on the side of an oil pan of the internal combustion engine the wall having a lubricant outlet hole formed therein, wherein lubricant that has been returned from the lubricant-return hole falls to the side of a lubricant storage of the oil pan through the lubricant outlet hole, wherein the lubricant outlet hole is shaped and positioned such that when the internal combustion engine is running in a steady manner, the level of a surface of lubricant stagnating between the ring gear and the outer race member does not exceed a horizontal line that is drawn tangent to the lower side of the outer periphery of the outer race member.
 16. The lubrication structure according to claim 15, wherein V14≦Cf·A15√{square root over ( )}(2gH) is true where V14 is a flow rate of lubricant flowing to the lubricant outlet hole, A15 is the sectional area of a lubricant passage in the lubricant outlet hole, H is the height from the lubricant outlet hole to a sealed slide surface of the sealing member at which the sealing member slidably contacts the outer race member, g is a gravitational acceleration, and cf is a flow rate coefficient of lubricant. 